Testing the Relationship of Brachiopod Body Size and Oxygen Levels during the Late Ordovician Mass Extinction

Abstract:

The Ordovician period, which initially saw a rise in biodiversity, was marked by a mass extinction event in its late stages known as the Late Ordovician Mass Extinction (LOME). We tested whether there was a preferential body size that led to higher extinction risk and if fluctuating atmospheric oxygen levels drove this extinction for brachiopods. For our research, we conducted analyses with the coding language R to create a logistic regression analysis and correlation tests to observe the relationship extinction risk has with body size and oxygen during each time stage of the Ordovician. Our study focused on three brachiopod classes: Rhynchonellata, Strophomenata, and Lingulata. The results, when focusing on the last two stages of the Ordovician when the LOME occurred, shows that smaller genera preferentially go extinct for Rhynchonellata and Strophomenata. We observed the opposite pattern for the class Lingulata, where the regression coefficients are positive during the LOME, indicating larger genera preferentially went extinct. For the correlation test between atmospheric oxygen (pO2) and the regression coefficients, the Rhynchonellata had the strongest negative correlation compared to the other two classes (Pearson’s r: -0.8537177 , p-value: 0.06567). Strophomenata and Lingulata extinction regression coefficients did not correlate with atmospheric oxygen, but there is stronger correlation when comparing their percent of extinction and pO2. However, these results were still not statistically significant (Pearson's r: 0.5436728 , p-value: 0.2648, Pearson’s r: 0.6880888, p-value: 0.1308, respectively). These results demonstrate that pO2 does not correlate with both extinction based on preferential body size and percent extinction, indicating factors other than oxygen might be playing a role. More data are needed for a conclusive result. In the future, we hope to collect more data and test more variables, such as sea level, ecology, available nutrients, or other environmental factors, across a larger range of brachiopod classes in order to uncover other factors that could have driven the extinction.